Alright I'll use 2.5. as a tester, I will try to find any and all possible problems.

I do love the summon system you got.
Multi-Slot Other - Line 30 Error.
I removed Skill learning script too.
I got this after I attacked a NPC then after like 3 seconds after he step back, the error appeared.
EDIT: I used your custom item script and pasted it in, and I got line 31 error.
So I suppose it doesn't work with this, as of right now. There error appeared when either I attacked the NPC, or the NPC attacked me. Though it was way after the attack like 2-3 seconds, hope I helped.
EDIT2: If you have a CMS with a face display feature the battle system will error could you try to have a different kind of display of faces using some other method.
Method and Class Library seems to not work with this, SDK is sort of compat with this, but causes some lag I think.
EDIT3: Could you code in a Battle Result script too? Something simple, just a add-on, because
the result music keeps playing. At-least I think.
Other than that this is the best CBS out right now.
The best thing I like about your CBS is that it keeps getting better, and the HP/SP bars look awesome. Best I have ever seen. That will be all from me for awhile, sorry for bugging you so much ><